IC-HK_1 Datasheet, PDF (5/13 Pages) IC-Haus GmbH – 155MHz LASER SWITCH

iC-HK
155MHz LASER SWITCH
Rev 21.11.01, Page 5/13
iC-WK‘s modulation range is at its maximum when for a current I(LDK) of ca. 45mA the voltage at pin CI vs. pin
GND of iC-WK is approximately 1.7V (min 1.1V, max 2.2V - limited by the saturation of the output stage or by the
overcurrent shutdown). The voltage across pin VCC and pin GND of iC-WK has to be sufficiently high so that
45mA through LDK will not drive the output stage into saturation.
A voltage of 1.7V at CI with RKx = 0S produces a current of approximately 150mA per channel through iC-HK‘s
pin LDK. For higher laser currents the voltage at CI can be increased by virtually raising iC-WK‘s pin GND. This
can be achieved by inserting a diode (DGND) or a resistor (RGND) between iC-WK's GND pin and the system
ground. The forward voltage of the diode V_fw(DGND) should satisfy the following condition:
V_fw(DGND) . V(CI) - 1.7V
Resistor RGND should be set to:
RGND . (V(CI) - 1.7V) / 45mA
Resistors RKx are not usually required. However for laser diodes operating on very low currents RKx might be
necessary due to the lower voltage limit of iC-WK at pin CI. When dimensioning the resistors Figures 2..4 from
the data sheet should be refered to as shown in Example 1. Furthermore, the use of resistors RKx can be useful
when implementing protection against overcurrent (see page 8, ‘Overcurrent shutdown/Laser current limitation’).
The value of capacitor CI depends primarily on the
pulse frequency. If CI is too small iC-WK would try to
readjust during a clock cycle and thus no control to
the mean would occur. Since iC-WK controls the
optical power by setting the voltage at pin CI,
overcurrent shutdown or - even worse - laser damage
might occur if the voltage at CI is too high with the
next high pulse. Capacitor CI must thus be sufficiently
large so that the voltage at CI remains more or less
constant during the low pulse. The following equation
is helpful when estimating a value for CI:
Fig. 11: Ripple at CI with respect to Im
CI $ (100 µA / f) / )V(CI)
Here, )V(CI) is the permissible ripple at CI and f the clock frequency. The permissible ripple depends on the
laser diode used; this is typically 2mV. For diodes with an extremely steep characteristic the permissible ripple
has to be reduced even further.
At low clock frequencies (<100kHz), high laser
currents or with an excessively high-resistance RM
the voltage at MDA may rise above ca. 0.7V during a
light pulse, thus triggering the permanent overcurrent
shutdown. In this instance we recommend using a
capacitor CM in parallel with RM. To avoid an
overshoot at pin CI when the system is switched on,
which can cause hazardous overcurrent to pass
through the laser, CM has to be selected so that the
time constant at node MDA is approximately 1/10 of
the CI control time constant:
CM . 1 / (10 × f × RM)
700mV
500mV
V(MDA)
700mV
500mV
V’(MDA)
Laser Output
Power
Should the use of CM be necessary capacitor CI has
to be increased to reduce a possible tendency
towards oscillation.
For 5mW laser power with a duty cycle t_hi/T of 1:10
and f = 100kHz, an Im_hi of 0.1mA is yielded for a
specific diode type, i.e. the mean monitor current is
Fig. 12: Voltage at MDA with (V(MDA)) and
without (V’(MDA)) capacitor CM